Method and system for hard handoff in a broadcast communication system

ABSTRACT

A method and system for a handoff in a broadcast communication system is disclosed. A subscriber-assisted handoff is impractical in a broadcast communication system due to e.g., a high signaling load, a difficulty to synchronize the broadcast transmission. On the other hand, the small number of broadcast channels enables the subscriber station to perform the handoff autonomously. While performing a handoff between broadcast transmissions, a disruption of service occurs due to lack of synchronization among the broadcast transmissions. To reduce the service disruption, upon detecting a need for handoff, the lower layers inform the application layer about the forthcoming handoff. The application layer thus may take measures to reduce or prevent the disruption.

BACKGROUND

[0001] 1. Field

[0002] The present invention relates to broadcast communications,otherwise known as point-to-multipoint communications, in a wirelesscommunication system. More particularly, the present invention relatesto a system and method for hard handoff in such a broadcastcommunication system.

[0003] 2. Background

[0004] Communication systems have been developed to allow transmissionof information signals from an origination station to a physicallydistinct destination station. In transmitting an information signal fromthe origination station over a communication channel, the informationsignal is first converted into a form suitable for efficienttransmission over the communication channel. Conversion, or modulation,of the information signal involves varying a parameter of a carrier wavein accordance with the information signal in such a way that thespectrum of the resulting modulated carrier is confined within thecommunication channel bandwidth. At the destination station the originalinformation signal is replicated using the received modulated carrierwave. Such a replication is generally achieved by using an inverse ofthe modulation process employed by the origination station.

[0005] Modulation also facilitates multiple-access, i.e., simultaneoustransmission and/or reception, of several signals over a commoncommunication channel. Multiple-access communication systems ofteninclude a plurality of subscriber units requiring intermittent serviceof relatively short duration rather than continuous access to the commoncommunication channel. Several multiple-access techniques are known inthe art, such as time division multiple-access (TDMA), frequencydivision multiple-access (FDMA), and amplitude modulationmultiple-access (AM). Another type of a multiple-access technique is acode division multiple-access (CDMA) spread spectrum system thatconforms to the “TIA/EIA/IS-95 Mobile Station-Base Station CompatibilityStandard for Dual-Mode Wide-Band Spread Spectrum Cellular System,”hereinafter referred to as the IS-95 standard. The use of CDMAtechniques in a multiple-access communication system is disclosed inU.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE-ACCESSCOMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and U.S.Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMSIN A CDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee ofthe present invention.

[0006] A multiple-access communication system may be a wireless orwire-line and may carry voice and/or data. An example of a communicationsystem carrying both voice and data is a system in accordance with theIS-95 standard, which specifies transmitting voice and data over thecommunication channel. A method for transmitting data in code channelframes of fixed size is described in detail in U.S. Pat. No. 5,504,773,entitled “METHOD AND APPARATUS FOR THE FORMATTING OF DATA FORTRANSMISSION”, assigned to the assignee of the present invention. Inaccordance with the IS-95 standard, the data or voice is partitionedinto code channel frames that are 20 milliseconds wide with data ratesas high as 14.4 Kbps. Additional examples of a communication systemscarrying both voice and data comprise communication systems conformingto the “3rd Generation Partnership Project” (3GPP), embodied in a set ofdocuments including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS25.213, and 3G TS 25.214 (the W-CDMA standard), or “TR-45.5 PhysicalLayer Standard for cdma2000 Spread Spectrum Systems” (the IS-2000standard).

[0007] An example of a data only communication system is a high datarate (HDR) communication system that conforms to the TIA/EIA/IS-856industry standard, hereinafter referred to as the IS-856 standard. ThisHDR system is based on a communication system disclosed in co-pendingapplication Ser. No. 08/963,386, entitled “METHOD AND APPARATUS FOR HIGHRATE PACKET DATA TRANSMISSION,” filed Nov. 3, 1997, and assigned to theassignee of the present invention. The HDR communication system definesa set of data rates, ranging from 38.4 kbps to 2.4 Mbps, at which anaccess point (AP) may send data to a subscriber station (accessterminal, AT). Because the AP is analogous to a base station, theterminology with respect to cells and sectors is the same as withrespect to voice systems.

[0008] In a multiple-access communication system, communications betweenusers are conducted through one or more base stations. A first user onone subscriber station communicates to a second user on a secondsubscriber station by transmitting data on a reverse link to a basestation. The base station receives the data and can route the data toanother base station. The data is transmitted on a forward link of thesame base station, or the other base station, to the second subscriberstation. The forward link refers to transmission from a base station toa subscriber station and the reverse link refers to transmission from asubscriber station to a base station. Likewise, the communication can beconducted between a first user on one subscriber station and a seconduser on a landline station. A base station receives the data from theuser on a reverse link, and routes the data through a public switchedtelephone network (PSTN) to the second user. In many communicationsystems, e.g., IS-95, W-CDMA, IS-2000, the forward link and the reverselink are allocated separate frequencies.

[0009] The above described wireless communication service is an exampleof a point-to-point communication service. In contrast, broadcastservices provide point-to-multipoint communication service. The basicmodel of a broadcast system consists of a broadcast net of users servedby one or more central stations, which transmit information with acertain contents, e.g., news, movies, sports events and the like to theusers. Each broadcast net user's subscriber station monitors a commonbroadcast forward link signal. Because the central station fixedlydetermines the content, the users are generally not communicating back.Examples of common usage of broadcast services communication systems areTV broadcast, radio broadcast, and the like. Such communication systemsare generally highly specialized purpose-build communication systems.With the recent, advancements in wireless cellular telephone systemsthere has been an interest of utilizing the existing infrastructure ofthe - mainly point-to-point cellular telephone systems for broadcastservices. (As used herein, the term “cellular” systems encompassescommunication systems utilizing both cellular and PCS frequencies.)

[0010] When a subscriber station travels outside the boundary of thebase station with which the subscriber station currently communicates,it is desirable to maintain the communication link by transferring thecall to a different subscriber station. The method and system forproviding a communication with a subscriber station through more thanone base station during the soft hand-off process are disclosed in U.S.Pat. No. 5,267,261, entitled “MOBILE ASSISTED SOFT HAND-OFF IN A CDMACELLULAR TELEPHONE SYSTEM,” assigned to the assignee of the presentinvention. The method and system for providing a softer hand-off isdescribed in detail in U.S. Pat. No. 5,933,787, entitled “METHOD ANDAPPARATUS FOR PERFORMING HAND-OFF BETWEEN SECTORS OF A COMMON BASESTATION”, assigned to the assignee of the present invention. Using thesemethods, communication between the subscriber stations is uninterruptedby the eventual handoff from an original base station to a subsequentbase station. This type of handoff may be considered a “soft” handoff inthat communication with the subsequent base station is establishedbefore communication with the original base station is terminated. Whenthe subscriber unit is in communication with two base stations, thesubscriber unit combines the signals received from each base station inthe same manner that multipath signals from a common base station arecombined.

[0011] Although the described handoff method for point-to-pointcommunication system described above could be applied to broadcastsystems, a handoff based on base station-subscriber station signalingmessage exchange would result in a high signaling load in a broadcastsystem. The high signaling load is caused by large number of subscribersmonitoring a common broadcast forward channel. Furthermore, as describedin the above-cited U.S. Pat. Nos. 5,267,261, and 5,933,787, thetransmissions received simultaneously by a subscriber station duringhandoff are synchronized at the transmitting base stations. Becausebroadcast transmission is intended for many subscriber stations, thebase station cannot synchronize transmission for each subscriber stationdesiring to handoff. Based on the foregoing, there is a need in the artfor a system and method for handoff in such a broadcast communicationsystem.

SUMMARY

[0012] Embodiments disclosed herein address the above stated needs byproviding a method and a system executing the method by detecting at aphysical layer a need for handoff; notifying higher layer about saiddetected need for handoff; and initiating re-synchronization inaccordance with said notifying.

[0013] In another aspect of the invention, once the notification wasreceived the initiating re-synchronization is carried out by determiningparameters for processing a second broadcast channel transmitted from asecond terminal; terminating processing of a first broadcast channeltransmitted from a first terminal; and beginning processing of thesecond broadcast channel in accordance with said determined parameters.

[0014] In yet another aspect of the invention, the output of a processedfirst broadcast channel is synchronized with the output of a processedsecond broadcast channel before the processing of a first broadcastchannel is terminated.

[0015] In yet another aspect of the invention, the above stated needsare addressed by providing a method and a system executing the method byprocessing a broadcast session on a first broadcast channel transmittedfrom a first terminal in accordance with a first set of parameters;performing a handoff to a second broadcast channel transmitted from asecond terminal; and processing a broadcast session on the secondbroadcast channel in accordance with the first set of parameters if ahandoff period is less than lifetime of the broadcast session.

[0016] In yet another aspect of the invention, the second set ofparameters are acquiring from the first broadcast channel or from thesecond broadcast channel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 illustrates conceptual block diagram of a High-SpeedBroadcast Service communication system;

[0018]FIG. 2 illustrates concept of soft-handoff groups in a broadcastcommunication system;

[0019]FIG. 3 illustrates re-synchronization in accordance with oneembodiment;

[0020]FIG. 4 illustrates broadcast stream re-alignment in accordancewith one embodiment;

[0021]FIG. 5 illustrates the broadcast stream re-alignment when the newbroadcast stream is advanced relative to the old broadcast stream;

[0022]FIG. 6 illustrates the broadcast stream re-alignment when the newbroadcast stream is delayed relative to the old broadcast stream;

[0023]FIG. 7 illustrates a Protocol Data Unit in accordance with oneembodiment; and

[0024]FIG. 8 illustrates re-synchronization in accordance with anotherembodiment.

DETAILED DESCRIPTION

[0025] Definitions

[0026] The word “exemplary” is used herein to mean “serving as anexample, instance, or illustration.” Any embodiment described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments.

[0027] The terms point-to-point communication is used herein to mean acommunication between two subscriber stations over a dedicatedcommunication channel.

[0028] The terms broadcast communication or point-to-multipointcommunication are used herein to mean a communication wherein aplurality of subscriber stations are receiving communication from onesource.

[0029] The term packet is used herein to mean a group of bits, includingdata (payload) and control elements, arranged into a specific format.The control elements comprise, e.g., a preamble, a quality metric, andothers known to one skilled in the art. Quality metric comprises, e.g.,a cyclical redundancy check (CRC), a parity bit, and others known to oneskilled in the art.

[0030] The term access network is used herein to mean a collection ofbase stations (BS) and one or more base stations' controllers. Theaccess network transports data packets between multiple subscriberstations. The access network may be further connected to additionalnetworks outside the access network, such as a corporate intranet or theInternet, and may transport data packets between each access terminaland such outside networks.

[0031] The term base station is used herein to mean the hardware withwhich subscriber stations communicate. Cell refers to the hardware or ageographic coverage area, depending on the context in which the term isused. A sector is a partition of a cell. Because a sector has theattributes of a cell, the teachings described in terms of cells arereadily extended to sectors.

[0032] The term subscriber station is used herein to mean the hardwarewith which an access network communicates. A subscriber station may bemobile or stationary. A subscriber station may be any data device thatcommunicates through a wireless channel or through a wired channel, forexample using fiber optic or coaxial cables. A subscriber station mayfurther be any of a number of types of devices including but not limitedto PC card, compact flash, external or internal modem, or wireless orwireline phone. A subscriber station that is in the process ofestablishing an active traffic channel connection with a base station issaid to be in a connection setup state. A subscriber station that hasestablished an active traffic channel connection with a base station iscalled an active subscriber station, and is said to be in a trafficstate.

[0033] The term physical channel is used herein to mean a communicationroute over which a signal propagates described in terms of modulationcharacteristics and coding.

[0034] The term logical channel is used herein to mean a communicationroute within the protocol layers of either the base station or thesubscriber station.

[0035] The term communication channel/link is used herein to mean aphysical channel or a logical channel in accordance with the context.

[0036] The term reverse channel/link is used herein to mean acommunication channel/link through which the subscriber station sendssignals to the base station.

[0037] A forward channel/link is used herein to mean a communicationchannel/link through which a base station sends signals to a subscriberstation.

[0038] The term hard handoff is used herein to mean a transfer of acommunication between a subscriber station and a first sector to asecond sector by ending the communication between the subscriber stationand the first sector before beginning the communication between asubscriber station and the second sector.

[0039] The term soft hand-off is used herein to mean a communicationbetween a subscriber station and two or more sectors, wherein eachsector belongs to a different cell. The reverse link communication isreceived by both sectors, and the forward link communication issimultaneously carried on the two or more sectors' forward links.

[0040] The term softer hand-off is used herein to mean a communicationbetween a subscriber station and two or more sectors, wherein eachsector belongs to the same cell. The reverse link communication isreceived by both sectors, and the forward link communication issimultaneously carried on one of the two or more sectors' forward links.

[0041] The term erasure is used herein to mean failure to recognize amessage.

[0042] The term dedicated channel is used herein to mean a channelmodulated by information specific to an individual subscriber station.

[0043] The term common channel is used herein to mean a channelmodulated by information shared among all subscriber stations.

[0044] Description

[0045] As discussed, a basic model of a broadcast system comprises abroadcast net of users, served by one or more central stations, whichtransmit information with a certain contents, e.g., news, movies, sportsevents and the like to the users. Each broadcast net user's subscriberstation monitors a common broadcast forward link signal. FIG. 1illustrates conceptual block diagram of a communication system 100,capable of performing High-Speed Broadcast Service (HSBS) in accordancewith embodiments of the present invention.

[0046] The broadcast content originates at a content server (CS) 102.The content server may be located within the carrier network (not shown)or outside Internet (IP) 104. The content is delivered in a form ofpackets to a broadcast packet data-serving node (BPDSN) 106. The termBPSDN is used because although the BPDSN may be physically co-located orbe identical to the regular PDSN (not shown), the BPSDN may be logicallydifferent from a regular PDSN. The BPDSN 106 delivers the packetsaccording to the packet's destination to a packet control function (PCF)108. The PCF is a control entity controlling function of base stations110 for the HSBS as a base station controller is for regular voice anddata services. To illustrate the connection of the high level concept ofthe HSBS with the physical access network, FIG. 1 shows a PCF physicallyco-located or even identical, but logically different from a basestation controller (BSC). One of ordinary skills in the art understandsthat this is for a pedagogical purposes only. The BSC/PCF 108 providesthe packets to base stations 110.

[0047] The communication system 100 enables High-Speed Broadcast Service(HSBS) by introducing a forward broadcast shared channel (F-BSCH) 112capable of high data rates transmitted from the base stations 110 thatcan be received by a large number of subscriber stations 114. The termforward broadcast shared channel is used herein to mean a single forwardlink physical channel that carries broadcast traffic. A single F-BSCHcan carry one or more HSBS channels multiplexed in a TDM fashion withinthe single F-BSCH. The term HSBS channel is used herein to mean a singlelogical HSBS broadcast session defined by the session's broadcastcontent. Each session is defined by a broadcast content that may changewith time; for example, 7am—News, 8am—Weather, 9am—Movies, etc.

[0048] Because as described, the HSBS channels are multiplexed onto aF-BSCH physical channel, and there are various possibilities for how theHSBS channels could be carried in the F-BSCH channels, the subscriberstation needs to know, which HSBS channel is carried on which F-BSCH.Such information is specified by a logical-to-physical mapping. Thephysical-to-logical mapping for broadcast services is disclosed in aco-pending U.S. patent application Ser. No. 09/933,978 entitled “AMETHOD AND SYSTEM FOR SIGNALING IN BROADCAST COMMUNICATION SYSTEM”,filed Aug. 20, 2001, and assigned to the assignee of the presentinvention. Furthermore, the forward broadcast shared channel comprisesvarious combinations of upper layer protocols, based on the type ofcontent being delivered. The subscriber station, therefore, furtherrequires information relating to these upper layer protocols forinterpretation of the broadcast transmissions.

[0049] The different options of arranging the HSBS services are referredto as HSBS service option. In general the HSBS service option is definedby a protocol stack, options in the protocol stack; and procedures forsetting up and synchronizing the service. The HSBS service option can beprovided to the subscriber station via out-of-band methods, i.e., viatransmission of the HSBS service option via a separate channel distinctfrom the broadcast channel. Alternatively, the HSBS service option canbe provided to the subscriber station via in-band methods, wherein theHSBS service option is multiplexed with the information content of theHSBS channel. The HSBS service option description can utilize protocolsknown to one of ordinary skills in the art. One of such protocoldescription of the application and transport layers comprises a SessionDescription Protocol (SDP). Session Description Protocol is a definedformat for conveying sufficient information to discover and participatein a multimedia or other broadcast type session. In one example, an SDPis specified in RFC 2327 entitled “SDP: Session Description Protocol” byM. Handley and V. Jacobson, dated April 1998, which is hereby expresslyincorporated by reference herein. Detailed description for providingprotocol options is disclosed in a co-pending U.S. patent applicationSer. No. 09/933,914 entitled “METHOD AND APPARATUS FOR BROADCASTSIGNALING IN A WIRELESS COMMUNICATION SYSTEM,” filed Aug. 20, 2001 , andassigned to the assignee of the present invention.

[0050] Hard Handoff on a Common Broadcast Forward Link

[0051] To enhance the Common Broadcast Forward Link performance, softand softer handoffs are desirable in overlapped coverage areas ofdifferent sectors. The method and system for providing a communicationwith a subscriber station through more than one base station during thesoft hand-off process are disclosed in a co-pending U.S. applicationSer. No. 09/933,607 , entitled “METHOD AND SYSTEM FOR A HANDOFF IN ABROADCAST COMMUNICATION SYSTEM,” filed on Aug. 20, 2001, and assigned tothe assignee of the present invention.

[0052] Although the described soft and softer handoff method isdesirable because the subscriber station does not experience adiscontinuity in transmitted information, such methods cannot always beutilized in a broadcast communication system. A subscriber station maysoft-combine only synchronous transmissions; consequently, thesubscriber station may perform a soft and softer handoff only betweenbase stations that belong to the same soft handoff (SHO) group. As usedherein a SHO group means a group of all Base Stations transmitting theCommon Broadcast Forward Link simultaneously and synchronously. FIG. 2illustrates two SHOs, one comprising BS₁, BS₂, and BS₃, the othercomprises BS₄, BS₅, BS₆, and BS₇. Consequently, if the subscriberstation crosses boundaries from a coverage area of SHO Group 1 202 to acoverage area of SHO Group 2 204, a hard handoff is required.

[0053] When the subscriber station performs a hard handoff between twocells/sectors that are not synchronized, e.g., BS₂ and BS₄, thebroadcast stream of the BS₂ may be advanced or delayed with respect tothe broadcast stream of the BS₄. Furthermore, because a handing off fromone broadcast stream to another broadcast stream takes some time, thebroadcast session, consequently, a service option, may change. Toprevent the subscriber station from incorrectly decoding the broadcaststream of the BS₄ the subscriber station needs to performre-synchronization procedures when such a hard handoff occurs. The termre-synchronization as used means processing necessary to provide userwith outputting of the broadcast service content after the handoff.During the re-synchronization procedures after a hard handoff, thedecoding and outputting of the broadcast service content is interrupted.This interruption can be annoying to the user and should be eliminated,or at least minimized.

[0054] In accordance with known handoff methods as described in, e.g.,the above-referenced U.S. Pat. Nos. 5,267,261 and 5,933,787, the needfor handoff is detected by layer 1 of the International StandardsOrganization (ISO) Open System Interconnection (OSI) layering model, andthe handoff is supported by layer 3 of the OSI. These layers arehereinafter referred to as lower layers. Layers above layer 3 of the OSI(hereinafter higher layers) are not notified or involved in the handoff.In accordance with the embodiments of the present invention, when thesubscriber station determines a need for handoff at the lower layers,e.g., the physical layer, the higher layer are notified about theupcoming handoff. Upon receiving the notification, the higher layersinitiate re-synchronization method.

[0055]FIG. 3 illustrates re-synchronization in accordance with oneembodiment. The method starts in step 3200 and continues in step 3202.

[0056] In step 3202, the need for hard handoff is tested. If the test ispositive, the method continues in step 3204, otherwise, the methodcontinues in step 3210.

[0057] In step 3204, the subscriber station, which is receiving abroadcast session on a broadcast channel transmitted form a firstterminal (an old broadcast stream) acquires a broadcast session on abroadcast channel transmitted form a second terminal (new broadcaststream), acquires the new broadcast stream, and searches the newbroadcast stream for the service option information, e.g., a headercompression protocol and decompression state, and session descriptionprotocol (SDP), while simultaneously decoding the full content of theold broadcast stream. Because the subscriber station does not need todecode the session content of the new stream, the subscriber stationdoes not to expand significant processing power. Once the subscriberstation identifies the service option information, the method continuesin step 3206.

[0058] In step 3206, the subscriber station decodes the identifiedservice option information, thus, acquiring the parameters necessary forprocessing the new broadcast stream. The method continues in step 3208.

[0059] In step 3208, the subscriber station concludes the handoff byending decoding and outputting the old broadcast stream. The methodcontinues in step 3210.

[0060] In step 3210, the subscriber station decodes and outputs thereceived broadcast stream, using the most recent parameters.

[0061] As discussed above, the old broadcast stream and the newbroadcast stream are not synchronized, which may cause discontinuity inthe output information. Furthermore, the time interval between theending decoding and outputting the old broadcast stream and startingdecoding and outputting the new broadcast stream may cause discontinuityin the output information. To minimize or prevent such discontinuities,in another embodiment, the subscriber station further determines thetiming of the old broadcast stream and the new broadcast stream and usethis information to re-align outputting the information content.

[0062]FIG. 4 illustrates re-alignment in accordance with one embodiment.The method starts in step 4300 and continues in step 4302.

[0063] In step 4302, upon detecting the need for handoff, the subscriberstation acquires the new broadcast stream, identifies and decodesservice option information for the new stream. The method of step 4302may comprise steps 3204 and 3206 of FIG. 3. The method continues in step4304.

[0064] In step 4304, the subscriber station further decodes timinginformation of the new broadcast stream. The timing information can beacquired for example from a time stamp of broadcast stream informationcontent. Alternatively, the timing information can be acquired from asequence numbers of broadcast stream information units. The methodcontinues in step 4306.

[0065] In step 4306, the subscriber station determines whether thebroadcast streams are synchronized. If the broadcast streams aresynchronized, the method continues in step 4308; otherwise, the methodcontinues in step 4310.

[0066] In step 4308, the subscriber station starts decoding the newbroadcast stream, and when the subscriber station is ready to output thedecoded new broadcast stream, the subscriber station concludes thehandoff by ending decoding and outputting the old broadcast stream andstarting outputting the new broadcast stream. The method returns to step4302.

[0067] In step 4310, the subscriber station determines the relativetiming of the old broadcast stream and the new broadcast stream. Furtherre-alignment depends on the relative timing of the broadcast streams.After re-aligning the broadcast streams, the subscriber stationconcludes the handoff, and the method returns to step 4302.

[0068]FIG. 5 illustrates the scenario, when the new broadcast stream isadvanced relative to the old broadcast stream. At time t₀, thesubscriber station is decoding the information content of the oldbroadcast stream 5402, and outputting the decoded information content5404. Simultaneously, the subscriber station decodes the informationcontent of the new broadcast stream 5406, and determines timingindicated by sequence numbers of units of the new broadcast stream 5406.At time t₁, the subscriber station starts accumulating units of the newbroadcast stream 5406 into a buffer 5408. At time t₂ the first unitstored in the buffer 5408 has the same sequence number (5) as the unitof the old broadcast stream 5402 to be outputted. Consequently, thesubscriber station starts outputting the units from the buffer 5408, anddiscontinues decoding and receiving of the old broadcast stream 5404. Toeliminate the delay between the content of the buffer 5408 and the unitsbeing received and decoded from the new broadcast stream 5406, thesubscriber station outputs the buffered units faster then the units arebeing received. In time t₃, there are no more units in the buffer 5408and the new broadcast stream 5406 unit (17) is aligned with the expectedunit (17) at the output 5404, consequently, the subscriber stationdiscontinues the units buffering, and provides the units directly to theoutput.

[0069]FIG. 6 illustrates the scenario, when the new broadcast stream isdelayed relative to the old broadcast stream. At time t₀, the subscriberstation is decoding the information content of the old broadcast stream6502, and outputting the decoded information content 6504.Simultaneously, the subscriber station decodes the information contentof the new broadcast stream 6506, and determines timing indicated bysequence numbers of units of the new broadcast stream 6506. At time t₁,the subscriber station starts accumulating units of the old broadcaststream 6502 into a buffer 6508, and outputs the units from the buffer6508 slower then the units from the new broadcast stream 6506 are beingreceived. At time t₂, the last unit stored in the buffer 6508 has thesame sequence number (13) as the unit of the new broadcast stream 6506to be outputted. Consequently, the subscriber station stops outputtingthe units from the buffer 6508, discontinues decoding and receiving ofthe old broadcast stream 6502, and starts outputting units form the newbroadcast stream 6506.

[0070] Usually, the two content streams have identical protocol options.The only time, when the protocol options are different before and afterhandoff is when the subscriber station performs a handoff during thetime the session changes. In another embodiment, the subscriber stationtakes advantage of the fact that the old stream may contain informationabout the next session.

[0071] As described in the above-referenced co-pending U.S. applicationSer. No. 09/933,914, the session description protocol (SDP), describingthe broadcast session, may be transmitted as a Protocol Data Unit (PDU),including multiple predefined fields as illustrated in FIG. 7. Thelength of the fields are given according to one embodiment, but may bevaried according to the design goals and constraints of a given system.The description of the PDU fields is as follows:

[0072] CONTROL identifies the format of the PDU, and indicates whetherfields NEXT_SDP_ID, INCL_SDP_DESC_ID, and INCL_SDP_DESC are included inthe PDU.

[0073] CURRENT_SDP_ID identifies the currently active SDP description,i.e., the description currently used for encoding and processing thebroadcast session content.

[0074] CURRENT_SDP_LIFE indicates a time for which the current SDP isvalid.

[0075] NEXT_SDP_ID identifies SDP for the next session. The can use thisID to retrieve the SDP of the next session before the next sessionstarts.

[0076] INCL_SDP_DESC_ID identifies the SDP that may be included in thePDU.

[0077] The SDP may be the current description, SDP for the next session,or SDP for any future sessions. Sending SDPs for future sessions allowsthe subscriber station to store the SDPs for viewing the future contentwithout retrieving the SDP directly from the server.

[0078] INCL_SDP_DESC the SDP for a particular session. Sending this SDPprevents subscriber stations from having to individually retrieve theSDP description from the content server. However, the SDP requires highbandwidth, it is recommended that the INCL_SDP_DESC is sent only priorto and after the session parameters have changed (i.e., at the boundarybetween two sessions).

[0079] The embodiment, utilizing the indication of a time for which thecurrent broadcast session is valid, starts in step 8600 and continues instep 8602.

[0080] In step 8602, the subscriber station processes the old broadcaststream, decodes service option information and determines a value of alife of the session, and whether the service option informationcomprises protocol parameters for the next session. The method continuesin step 8604.

[0081] In step 8604, the subscriber station compares the value of thelife of the session with a handoff time. If value of the life of thesession is greater that the handoff time, the method continues in step8606, otherwise, the method continues in step 8608.

[0082] In step 8606, the subscriber station performs handoff, acquiresthe new stream, and starts processing the new broadcast stream with thecurrent service option information. The method returns to step 8602.

[0083] In step 8608, the subscriber station continues in accordance withdetermination whether the service option information comprises protocolparameters for the next session as determined in step 8602. If theservice option information comprises protocol parameters for the nextsession, the method continues in step 8610; otherwise, the methodcontinues in step 8612.

[0084] In step 8610, the subscriber station performs handoff, acquiresthe new stream, and starts processing the new broadcast stream with theprotocol parameters for the next session.

[0085] In step 8612, the subscriber station performs handoff, acquiresthe new stream, identifies and decodes service option information, anddecodes the new broadcast stream with the protocol parameters acquiredfrom the service option information. The method continues in step 8602.

[0086] One skilled in the art will appreciate that although theflowchart diagrams are drawn in sequential order for comprehension,certain steps can be carried out in parallel in an actualimplementation. Additionally, unless indicate otherwise, method stepscan me interchanged without departing form the scope of the invention.Furthermore, although the signaling of the service option was describedin terms of in-band signaling, this was for tutorial purposes only, anduse of out-of-band signaling is within the spirit and scope of thepresent invention.

[0087] Those of skill in the art would understand that information andsignals may be represented using any of a variety of differenttechnologies and techniques. For example, data, instructions, commands,information, signals, bits, symbols, and chips that may be referencedthroughout the above description may be represented by voltages,currents, electromagnetic waves, magnetic fields or particles, opticalfields or particles, or any combination thereof.

[0088] Those of skill would further appreciate that the variousillustrative logical blocks, modules, circuits, and algorithm stepsdescribed in connection with the embodiments disclosed herein may beimplemented as electronic hardware, computer software, or combinationsof both. To clearly illustrate this interchangeability of hardware andsoftware, various illustrative components, blocks, modules, circuits,and steps have been described above generally in terms of theirfunctionality. Whether such functionality is implemented as hardware orsoftware depends upon the particular application and design constraintsimposed on the overall system. Skilled artisans may implement thedescribed functionality in varying ways for each particular application,but such implementation decisions should not be interpreted as causing adeparture from the scope of the present invention.

[0089] The various illustrative logical blocks, modules, and circuitsdescribed in connection with the embodiments disclosed herein may beimplemented or performed with a general purpose processor, a digitalsignal processor (DSP), an application specific integrated circuit(ASIC), a field programmable gate array (FPGA) or other programmablelogic device, discrete gate or transistor logic, discrete hardwarecomponents, or any combination thereof designed to perform the functionsdescribed herein. A general purpose processor may be a microprocessor,but in the alternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

[0090] The steps of a method or algorithm described in connection withthe embodiments disclosed herein may be embodied directly in hardware,in a software module executed by a processor, or in a combination of thetwo. A software module may reside in RAM memory, flash memory, ROMmemory, EPROM memory, EEPROM memory, registers, hard disk, a removabledisk, a CD-ROM, or any other form of storage medium known in the art. Anexemplary storage medium is coupled to the processor such the processorcan read information from, and write information to, the storage medium.In the alternative, the storage medium may be integral to the processor.The processor and the storage medium may reside in an ASIC. The ASIC mayreside in a user terminal. In the alternative, the processor and thestorage medium may reside as discrete components in a user terminal.

[0091] The previous description of the disclosed embodiments is providedto enable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

[0092] A portion of the disclosure of this patent document containsmaterial which is subject to copyright protection. The copyright ownerhas no objection to the facsimile reproduction by anyone of the patentdocument or the patent disclosure, as it appears in the Patent andTrademark Office patent file or records, but otherwise reserves allcopyright rights whatsoever.

What is claimed is:
 1. A method for re-synchronization in acommunication system, the method comprising: detecting at a physicallayer a need for handoff; and notifying higher layer about said detectedneed for handoff.
 2. The method as claimed in claim 1, furthercomprising: determining parameters for processing a second broadcastchannel transmitted from a second terminal; terminating processing of afirst broadcast channel transmitted from a first terminal; and beginningprocessing of the second broadcast channel in accordance with saiddetermined parameters.
 3. The method as claimed in claim 2, furthercomprises: adjusting outputting of a processed first broadcast channeltransmitted from a first terminal in response to said beginningprocessing of the second broadcast channel.
 4. The method as claimed inclaim 3 wherein said adjusting outputting comprises: reducing a rate ofoutputting the processed first broadcast channel.
 5. The method asclaimed in claim 3 wherein said adjusting outputting comprises:increasing a rate of outputting the processed first broadcast channel.6. The method as claimed in claim 2, wherein said terminating processingof a first broadcast channel comprises: terminating processing of afirst broadcast channel upon synchronizing the first broadcast channeland the second broadcast channel.
 7. The method as in claim 6 whereinsaid synchronizing the first broadcast channel and the second broadcastchannel comprises: identifying a common time stamp in the firstbroadcast channel and the second broadcast channel.
 8. The method as inclaim 6, wherein said synchronizing the first broadcast channel and thesecond broadcast channel comprises: identifying a common sequence numberin the first and second transmission streams.
 9. A method forre-synchronization in a communication system, the method comprising:processing a first broadcast channel transmitted from a first terminal;determining parameters for processing a second broadcast channeltransmitted from a second terminal; terminating processing of the firstbroadcast channel; and beginning processing of the second broadcastchannel in accordance with said determined parameters.
 10. The method asclaimed in claim 9 further comprising: adjusting outputting of theprocessed first broadcast channel in response to said beginningprocessing of the second broadcast channel.
 11. The method as claimed inclaim 10, wherein said adjusting outputting comprises: reducing a rateof outputting the processed first broadcast channel.
 12. The method asclaimed in claim 10, wherein said adjusting outputting comprises:increasing rate of outputting of the processed first broadcast channel.13. The method as claimed in claim 9, wherein said terminatingprocessing of a first broadcast channel comprises: terminatingprocessing of a first broadcast channel upon synchronizing the firstbroadcast channel and the second broadcast channel.
 14. The method as inclaim 13 wherein said synchronizing the first broadcast channel and thesecond broadcast channel comprises: identifying a common time stamp inthe first broadcast channel and the second broadcast channel.
 15. Themethod as in claim 13, wherein said synchronizing the first broadcastchannel and the second broadcast channel comprises: identifying a commonsequence number in the first broadcast channel and the second broadcastchannel.
 16. The method as in claim 9, further comprising: detecting ata physical layer a need for a handoff; notifying higher layer about saiddetected need for handoff.
 17. A method for re-synchronization in acommunication system, the method comprising: processing a broadcastsession on a first broadcast channel transmitted from a first terminalin accordance with a first set of parameters; performing a handoff to asecond broadcast channel transmitted from a second terminal; andprocessing a broadcast session on the second broadcast channel inaccordance with the first set of parameters if a handoff period is lessthan life of the broadcast session.
 18. The method as in claim 17,further comprising: processing a broadcast session on the secondbroadcast channel in accordance with a second set of parameters if ahandoff period is greater than lifetime of the broadcast session. 19.The method as in claim 17, wherein said processing a broadcast sessionon the second broadcast channel in accordance with a second set ofparameters comprises: acquiring the second set of parameters from thefirst broadcast channel.
 20. The method as in claim 17, wherein saidprocessing a broadcast session on the second broadcast channel inaccordance with a second set of parameters comprises: acquiring thesecond set of parameters from the second broadcast channel.
 21. Themethod as in claim 17, further comprising: detecting at a physical layera need for a handoff; and notifying higher layer about said detectedneed for handoff.
 22. An apparatus for re-synchronization in acommunication system, the apparatus comprising: means for detecting at aphysical layer a need for handoff; and means for notifying higher layerabout said detected need for handoff.
 23. The apparatus as claimed inclaim 22, further comprising: means for determining parameters forprocessing a second broadcast channel transmitted from a secondterminal; means for terminating processing of a first broadcast channeltransmitted from a first terminal; and means for beginning processing ofthe second broadcast channel in accordance with said determinedparameters.
 24. The apparatus as claimed in claim 23, further comprises:means for adjusting outputting of a processed first broadcast channeltransmitted from a first terminal in response to said beginningprocessing of the second broadcast channel.
 25. The apparatus as claimedin claim 24 wherein said means for adjusting outputting comprises: meansfor reducing a rate of outputting the processed first broadcast channel.26. The apparatus as claimed in claim 24 wherein said means foradjusting outputting comprises: means for increasing a rate ofoutputting the processed first broadcast channel.
 27. The apparatus asclaimed in claim 23, wherein said means for terminating processing of afirst broadcast channel comprises: means for terminating processing of afirst broadcast channel upon synchronizing the first broadcast channeland the second broadcast channel.
 28. The apparatus as in claim 27wherein said synchronizing the first broadcast channel and the secondbroadcast channel comprises: identifying a common time stamp in thefirst broadcast channel and the second broadcast channel.
 29. Theapparatus as in claim 27, wherein said synchronizing the first broadcastchannel and the second broadcast channel comprises: identifying a commonsequence number in the first and second transmission streams.
 30. Anapparatus for re-synchronization in a communication system, theapparatus comprising: means for processing a first broadcast channeltransmitted from a first terminal; means for determining parameters forprocessing a second broadcast channel transmitted from a secondterminal; means for terminating processing of the first broadcastchannel; and beginning processing of the second broadcast channel inaccordance with said determined parameters.
 31. The apparatus as claimedin claim 30 further comprising: means for adjusting outputting of theprocessed first broadcast channel in response to said beginningprocessing of the second broadcast channel.
 32. The apparatus as claimedin claim 31, wherein said means for adjusting outputting comprises:means for reducing a rate of outputting the processed first broadcastchannel.
 33. The apparatus as claimed in claim 31, wherein said meansfor adjusting outputting comprises: means for increasing rate ofoutputting of the processed first broadcast channel.
 34. The apparatusas claimed in claim 30, wherein said means for terminating processing ofa first broadcast channel comprises: means for terminating processing ofa first broadcast channel upon synchronizing the first broadcast channeland the second broadcast channel.
 35. The apparatus as in claim 34wherein said synchronizing the first broadcast channel and the secondbroadcast channel comprises: identifying a common time stamp in thefirst broadcast channel and the second broadcast channel.
 36. Theapparatus as in claim 34, wherein said synchronizing the first broadcastchannel and the second broadcast channel comprises: identifying a commonsequence number in the first broadcast channel and the second broadcastchannel .
 37. The apparatus as in claim 30, further comprising: meansfor detecting at a physical layer a need for a handoff; and means fornotifying higher layer about said detected need for handoff.
 38. Aapparatus for re-synchronization in a communication system, theapparatus comprising: means for processing a broadcast session on afirst broadcast channel transmitted from a first terminal in accordancewith a first set of parameters; means for performing a handoff to asecond broadcast channel transmitted from a second terminal; and meansfor processing a broadcast session on the second broadcast channel inaccordance with the first set of parameters if a handoff period is lessthan life of the broadcast session.
 39. The apparatus as in claim 38,further comprising: means for processing a broadcast session on thesecond broadcast channel in accordance with a second set of parametersif a handoff period is greater than lifetime of the broadcast session.40. The apparatus as in claim 38, wherein said means for processing abroadcast session on the second broadcast channel in accordance with asecond set of parameters comprises: means for acquiring the second setof parameters from the first broadcast channel.
 41. The apparatus as inclaim 38, wherein said means for processing a broadcast session on thesecond broadcast channel in accordance with a second set of parameterscomprises: means for acquiring the second set of parameters from thesecond broadcast channel.
 42. The apparatus as in claim 38, furthercomprising: means for detecting at a physical layer a need for ahandoff; and means for notifying higher layer about said detected needfor handoff.
 43. An apparatus for providing a multiple layer content,comprising: a memory; and a device communicatively coupled to the memoryand capable of performing digital signal processing including: detectingat a physical layer a need for handoff; and notifying higher layer aboutsaid detected need for handoff.
 44. The apparatus of claim 43, thedevice communicatively completed to the memory and capable of performingdigital signal processing further includes initiating re-synchronizationin accordance with said notifying.
 45. The apparatus of claim 44 whereinsaid initiating re-synchronization comprises: determining parameters forprocessing a second broadcast channel transmitted from a secondterminal; terminating processing of a first broadcast channeltransmitted from a first terminal; and beginning processing of thesecond broadcast channel in accordance with said determined parameters.46. An apparatus for providing a multiple layer content, comprising: amemory; and a device communicatively coupled to the memory and capableof performing digital signal processing including: processing a firstbroadcast channel transmitted from a first terminal; determiningparameters for processing a second broadcast channel transmitted from asecond terminal; terminating processing of the first broadcast channel;and beginning processing of the second broadcast channel in accordancewith said determined parameters.
 47. An apparatus for providing amultiple layer content, comprising: a memory; and a devicecommunicatively coupled to the memory and capable of performing digitalsignal processing including: processing a broadcast session on a firstbroadcast channel transmitted from a first terminal in accordance with afirst set of parameters; performing a handoff to a second broadcastchannel transmitted from a second terminal; and processing a broadcastsession on the second broadcast channel in accordance with the first setof parameters if a handoff period is less than lifetime of the broadcastsession.